In recent years, a technology for forming a thin film transistor (TFT) using a semiconductor thin film (a thickness of around several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has been attracting attention. Thin film transistors are broadly applied to electronic devices such as an IC or an electro-optic device, and are particularly developed as switching elements for image display devices at a rapid rate.
Above all, a thin film transistor (TFT) formed using a polycrystalline semiconductor film is higher in mobility by two digits or more than a TFT formed using an amorphous semiconductor film, and has an advantage that a pixel portion and a peripheral driver circuit in a semiconductor display device can be formed to be integrated over the same substrate. The polycrystalline semiconductor film can be formed over an inexpensive glass substrate by employing a laser annealing method.
As laser oscillators used in the laser annealing method, there are two types, a pulsed laser oscillator and a continuous wave laser oscillator, according to the oscillation method. The pulsed laser typified by an excimer laser has output power per unit time which is approximately three to six digits higher than that of the continuous wave laser. Therefore, the throughput of the laser irradiation can be increased by shaping a beam spot (a region irradiated with the laser beam in fact on the surface of an object to be irradiated) into a rectangular spot having a length of several cm on a side or a linear spot having a length of 100 mm or more with the use of an optical system to efficiently conduct laser irradiation to a semiconductor film. For this reason, the pulsed laser has been mainly employed for crystallizing the semiconductor film.
It is noted that a term of “linear” herein used does not mean a line in a strict sense but means a rectangle (or elongated ellipse) having a large aspect ratio. For example, a rectangular beam spot having an aspect ratio of 2 or more (preferably 10 to 10000) is referred to as a linear beam spot. It is to be noted that the linear is included in the rectangular.
Reference 1 (Japanese Patent No. 3276900) describes a technology by which, as to two channel-forming regions (CH) each having a channel width W to become an operation design value, the total value of W and an interval distance WA between the two channel-forming regions is made larger than a pitch P of a pulsed laser, using the pulsed laser.